This invention relates to methods and compositions for forming blown extruded thermoplastic foams. More particularly, this invention relates to improved methods and compositions for forming blown extruded thermoplastic foams which methods and compositions advantageously avoid the use of blowing agents believed to be harmful to the Ozone layer of the stratosphere.
It is known in the art of foam products that thermoplastic foams may be produced by extruding a molten mass of thermoplastic material mixed with an appropriate blowing agent which is volatile at the temperature of the extrusion. The blowing agent is thoroughly mixed with the molten thermoplastic mass under high pressure. The molten mass is then extruded into a region of lower pressure, whereby the volatile blowing agent expands to form cells in the mass, resulting in a cellular structure called foam. The foam may be extruded by a screw extruder through an annular die, such that the foam is extruded as an elongated tubular body which may then be slit into sheets.
These volatile blowing agents are generally of a type which are sufficiently soluble in the thermoplastic material and have a diffusion rate through the thermoplastic material sufficient to provide a controlled rate of expansion upon extrusion. It is recognized in the art that foam bodies made by such methods and compositions may have to be aged prior to being formed into useful articles, in order to first allow a part of the residual volatile blowing agent to diffuse out of the foam and air to diffuse into the cells. In addition, any remaining blowing agent can result in post-extrusion expansion of the foam when articles are subsequently manufactured by heat forming methods. It is known that the choice of volatile blowing agent may vary with the particular thermoplastic resin being extruded. For some thermoplastics, it is desirable that the volatile blowing agent also act as a plasticizer to facilitate extrusion of the molten mass. These volatile blowing agents are generally selected because they are readily available and inexpensive.
Common blowing agents of the prior art include organic solvents which are highly volatile at extrusion temperatures. These organic solvents typically include the simple alkanes such as butane, isobutane, pentane, hexane, and the like. These blowing agents readily volatilize under extrusion conditions, and also provide an incidental plasticizing effect with many common thermoplastic materials used to make foams.
Certain halogenated hydrocarbons also have become popular as blowing agents, particularly in the extrusion of polystyrene foam. These halogenated hydrocarbon blowing agents include many of the chlorofluorocarbon compounds such as trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), and 1,1-dichloro-2,2,2-trifluoroethane, as well as methylene chloride and 1,1,1-trichloroethane. The halogenated hydrocarbons also have the beneficial incidental plasticizing effects associated with the alkane blowing agents.
Recent speculation, however, has suggested that the release of some of these compounds into the atmosphere may be damaging to the environment, particularly to the Ozone layer of the earth's stratosphere. For example, CFC-11 and CFC-12 have not been used as blowing agents since about 1988, and under the Clean Air Act the use of HCFC-22 as a foam blowing agent is being phased out by the end of the year 1993. Elimination of the use of these volatile agents in industrial processes in general, and in the production of thermoplastic foam in particular, has been the target of this work.
Prior attempts have been made to substitute non-polluting blowing agents for at least part of the commonly used organic blowing agents. Atmospheric gases such as nitrogen and carbon dioxide have been suggested as alternative blowing agents. Generally, the use of atmospheric gases as blowing agents has posed significant problems in the control of the extrusion and foaming processes. The lack of plasticizing effect of the foaming agent results in a more viscous extrudate, requiring the extruder to operate at greater torque and at higher temperatures, resulting in undesirable heat transfer to the extrudate. In addition, the very low solubility of the atmospheric gases in common thermoplastics and their rapid rate of diffusion through thermoplastic materials create problems in control of foaming in the extrudate. Loss of control in the production of foam at commercial rates can result in non-homogeneous cell size, burst cells and foam of uneven thickness and density. Each of these results can lead to weaknesses in the foam structure rendering it unacceptable for subsequent use in the manufacture of finished articles.
It has been found that the foregoing problems can be overcome, and acceptable foam can be made at commercial production rates, when CO.sub.2 or another atmospheric gas is used as the blowing agent and the thermoplastic mass is provided with an effective amount of a non-fugitive plasticizing agent and a nucleating agent. This method is disclosed in the co-pending application Ser. No. 819,613, filed on Jan. 9, 1992, based on a parent application Ser. No. 497,731 filed Mar. 19, 1990, and a grandparent application Ser. No. 194,230 filed May 11, 1988, assigned to the common assignee herein, and incorporated herein by reference in its entirety. The use of gaseous CO.sub.2 blowing agent as disclosed in that copending application has been found to make foam suitable for the manufacture of a variety of products, and in particular foam beverage cups and labels.
Because of its low solubility and high rate of diffusion in molten thermoplastic and thermoplastic foam, however, gaseous CO.sub.2 foam has been less satisfactory as a blowing agent in the manufacture of low density foams, such as those in the density range of about 4 lbs/ft.sup.3. Such low density foams are typically used for applications such as foam tableware, including foam plates and bowls. In prior attempts to make low-density foam at commercial output rates, it has been found that the use of pure CO.sub.2 blowing agent, even with an effective amount of a non-fugitive plasticizing agent and nucleating agent, results in low-density foam having unacceptable surface corrugations.
One hydrocarbon blowing agent which is believed to have no undesirable Ozone depletion potential (ODP) is 1,1-difluoroethane, or CH.sub.3 CHF.sub.2, also known in the industry as Formacel Z-2. As disclosed in U.S. Pat. No. 5,147,896, issued Sep. 15, 1992 to York and assigned to E. I. dupont de Nemour and Company, Formacel Z-2 has been proven satisfactory as a blowing agent for low-density thermoplastic foam. In fact, Formacel Z-2 can produce foam of the same low density as that produced with HCFC-22, but with using about 20-25% less Formacel Z-2 than HCFC-22, on a mass basis. This is because the molecular weight of HCFC-22 is 86, while the molecular weight of Formacel Z-2 is only 66; i.e., the molecular weight of Formacel Z-2 is about 25% less than the molecular weight of HCFC-22. Thus, pound for pound, Formacel Z-2 is more efficient as a blowing agent than HCFC-22. Even with this greater efficiency, however, Formacel Z-2 is expensive to use as a blowing agent for foam production on an industrial scale.